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Friday, January 16, 2015

The sandstone that fooled the great geologists (but the younger generation needs to understand)

Flaming Gorge Reservoir from the canyon rim.

The Uinta Mountains of northeast Utah and adjacent Wyoming are “a grand sandstone structure” – red sandstone peaks dot the high country, tilted beds are exposed on the flanks, and the Green River flows through the range via narrow canyons with steep sandstone walls. In 1964 Flaming Gorge Reservoir flooded these canyons, once home of the Ute people. They say that at night their spirits walk the rims, staring at the water 1300 feet below, trying to rescue drowning memories.

I camped on the rim last fall and sure enough, around midnight two voices woke me from a deep sleep. But they weren’t speaking Ute.

the broad, massive range is a grand sandstone structure ... considering this formation to be older than Carboniferous, I have given it provisionally a Devonian color on the map1

they are quite conformable with the limestones, which abound in well-deﬁned fossil remains, so they have been referred to the Carboniferous Weber Quartzite group2

I peeked out of the tent but saw only trees and stars. “Must be dreaming” I thought but then there was a third voice, barely audible above the wind in the ponderosa pines:

I strongly suspect the purplish sandstones to be of Lower Silurian age. The precise or approximate age of these rocks is a very interesting problem to me, and I regret very much that my time would not permit me to make a thorough exploration of both sides of this most interesting range3

This was followed by a loud and sharp retort:

Your geology is hopeless and your private character bad!4

Tussling and moaning ensued. Maybe it was just the dog and her blanket but I couldn’t be sure. However if these were indeed visitors, I now knew who they were – John Wesley Powell, Samuel Franklin Emmons, Ferdinand Vandeveer Hayden and Clarence King – four great 19th-century geologists of the American West. All had explored the Uinta Mountains. Now their spirits walked the canyon rims, still debating the age of the rocks below.

“Silurian!” “Carboniferous!!” “Devonian!!!” ... it was a spirited debate!

After the Civil War ended in 1865, American eyes looked west toward the country’s unknown territory – from the Rocky Mountains to the east side of the Sierra Nevada. Government, commercial interests and individuals all hoped it was a land of riches; rumors, some grossly inflated, fueled the fire. But no one really knew what was out there.

Four highly-motivated men with strong personalities and good connections saw this as an opportunity. Eachsecured funding from the US government to explore, describe and map large parts of the terra incognita. These were the Great Surveys. They overlapped extensively in time and space, but were carried out independently and with more than a little competition and acrimony ... not surprising given the personalities involved. The area was so large and existing knowledge so limited that the "surveys" were perhaps better called reconnaissances or explorations. Even so they generated massive amounts of knowledge where there was little before, and produced maps of sufficient detail and quality to route railroads, choose farm land, and narrow the search for riches (Bartlett 1962; Hansen 1969, Introduction).

Three of the four surveys included the Uinta Mountains (the exception was the Wheeler Survey). Each produced a geologic map, and each had to assign an age to the sandstone that dominated the range. The latter task proved to be especially challenging.

At that time geologists generally dated rocks based on fossils of known age, or based on the rocks’ position relative to others of known age (we assume younger rock strata lie atop older unless there’s been a subsequent disruption). Unfortunately survey geologists found no fossils in the sandstone that formed the core of the Uintas, and its position relative to known strata proved to be misleading. In some cases they thought they could correlate the sandstone with rock units elsewhere. That also got them into trouble.

Ferdinand Vandeever Hayden led the US Geological and Geographical Survey of the Territories. He had a reputation for wide-ranging but superficial study; some even considered him more of a self-promoting lobbyist than a geologist. But he productively covered an immense amount of territory, including Nebraska, Wyoming, Colorado and parts of Utah. In the late summer of 1870, he spent time on the northern slope of the Uinta Mountains where he struggled to understand extensive outcrops of "dull purplish" sandstone.

The succeeding series of rocks I have not found it so easy to locate in the geological scale. They consist of dull, purplish sandstones, with a series of thin layers of slate and clay, gradually passing down into quartzites(Hayden 1871).

Hayden was a staunch “layer-cake” geologist (Aalto 2005), believing deposition to be continuous through time to produce an uninterrupted sequence of rocks. He was not inclined to see unconformities – gaps in the rock record due to erosion or a hiatus in deposition.

In all this series of strata, from the red-beds to the oldest quartzites, I was able to detect no unconformability. The connection of the [dull purplish]sandstones with the carboniferous limestones [above]was perfect, so far as could be ascertained by the eye, whatever may have been the chasm in time.

Based on similarities with rocks elsewhere, Hayden tentatively concluded that the sandstone at the core of the Uintas dated from the Silurian period ...

The texture of the upper beds of sandstone is so much like the Potsdam sandstones, as may be observed in other portions of the Rocky Mountain region, I was lead to suspect that the upper portion might be Lower Silurian.

... yet he also noted its uniqueness.

I have never observed such a series of rocks in any other portion of the West, and am inclined to think they are confined to the Uinta range. The Uinta Mountains are not far from the Wasatch range, and apparently join on to that range; yet I have passed through the Wasatch range at right angles at different points, and was able to discover no such series of strata.

Clarence King headed the US Geological Exploration of the 40th Parallel. As well as the usual guides, hunters and other support staff, he enlisted topographers, geologists, zoologists, botanists, paleontologists and more in order to glean every bit of information possible. King divided his expedition into crews, each with a topographer and geologist. The Uinta Mountains were assigned to geologist Samuel Franklin Emmons.

The Uinta sandstones puzzled Emmons too. He referred to them as quartzites. Both are present; the quartzite tends towards sandstone, its precursor.

They consist of a lower series of white and reddish, compact quartzites, a middle series of purple, coarse quartzites, and an upper of red and striped sandstones ... they are in general barren of all fossil remains (Emmons 1877).

Directly above the quartzites and sandstones was a layer of limestone known to be Carboniferous in age. Had deposition been continuous, or was there a gap in the rock record – an unconformity – between the sandstones and the limestone above? Emmons found no clear evidence of an unconformity, and concluded that the Uinta quartzites and sandstones could be correlated with the Carboniferous Weber Quartzite of the Wasatch Mountains to the west.

as they are quite conformable with the limestones of the Upper Coal-Measure group, which abound in well-deﬁned fossil remains, they have been referred to the Weber Quartzite group.

Emmons had some reservations about correlating the quartzite/sandstone with the Weber.

Their thickness[Uinta quartzites], as observed in different parts of the range, is from 10,000 to 12,000 feet, while, as their base is never reached, the actual thickness of the formation may be indeﬁnitely greater. This fact might seem to throw some doubt upon the correctness of the assumption that they all belong to the Carboniferous formation, since the Weber Quartzite, in the Wahsatch Range, at a comparatively short distance to the west, attains a development of only 5,000 to 7,000 feet, and the general tendency of all the formations is to thin out to the eastward.

After he finished his writeup, Emmons learned of discoveries by Powell in the eastern Uintas that cast additional doubt on his correlation of the Uinta quartzite with the Weber. He was able to insert a footnote before the final report was published (Volume II, 1877).

Since the above was written, there has been published, in the report of Prof. J. W. Powell, on the Geology of the Eastern Uinta Mountains (p. 144), sections made in the cañons of the Green River, showing an unconformity of deposition in these beds, which, if correct, would seem to prove the latter supposition to be the more correct[i.e. the sandstone/quartzite is much older].

The best-known of the Great Survey leaders was John Wesley Powell, head of the US Geographical and Geological Survey of the Rocky Mountain Region. His pioneering descents of the Green and Colorado Rivers gained him fame far beyond geological circles. The Green took Powell through the Uinta Mountains, via deep canyons where he saw things not accessible to the other surveyors.

In the many deep cañons and gulches by which the range is cleft, in the many amphitheaters that are found along the crest of the range, and in the mural faces of its lofty peaks, everywhere the sandstones are made bare to the eye of the geologist (Powell 1876).

In Whirlpool Canyon, he saw the sandstones not only made bare, but also lying unconformably under Carboniferous limestone. The sandstone layer clearly had been eroded before the limey sediments were deposited.

Powell provided a diagram of the unconformity (at red arrow), showing hills and valleys in the eroded upper surface of the Uinta sandstone. Modified from Powell 1876.

The period of erosion separating the sandstones from the Carboniferous beds above was sufficient to carry away at least 3,000 feet of the Uinta Sandstone in some places.How much more was carried away we cannot say. To my mind this suggests that the Uinta Sandstone may be considered Devonian—an opinion which I would yield upon the slightest paleontologic evidence to the contrary.

Thus all of the survey geologists assigned the enigmatic Uinta sandstone to the middle of the Paleozoic era, though they disagreed on the exact period. But all were wrong. It's actually much older.

On three published geological maps they [quartzites and sandstones]have been assigned successively to as many different periods–on the [King] maps to the Carboniferous, on the Powell map to the Devonian, and on the Hayden maps to the Silurian–whereas in point of fact they do not belong to any one of the three (Emmons 1907).

A clear age for the Uinta sandstones proved elusive ... until Emmons took another look in 1906.

When he was 65 years old, Samuel Franklin Emmons returned to the Uinta Mountains and the puzzling quartzite/sandstone. In several canyons where rocks were well-exposed, he saw the unconformity described by Powell from Whirlpool Canyon. Based on correlation with rocks in the Wasatch Mountains, Emmons concluded that the Uinta rocks were in fact Precambrian.

The Uinta quartzites I regard as undoubtedly of Pre-Cambrian age, and, like most Pre-Cambrian formations in the West, which are widely separated and generally barren of fossils, its exact correlation will for a long time probably remain in doubt(Emmons 1907).

Roadcut in Precambrian sandstones near Flaming Gorge dam.

That winter, Emmons read a paper about his findings before the Geological Society of America. He took the opportunity to explain to the younger generation the challenges faced by geologists of the Great Surveys.

A generation has passed awaysince these maps were made, during which time the advance in geological knowledge of the West has been so great and the change in methods of work so radical that it is difficult for the younger generation of geologists to appreciate the conditions under which geological work was then done.

So large were the areas laid out for the field work of each season that the utmost speed was necessary in order to go over the whole ground before snow rendered geological work impracticable. Thus the Green River sheet, which covers an area of over 15,000 square miles, including the greater part of the Uinta mountains, was completed during the single field season of 1871.

The general system of work was to construct in our minds, from field observations, a tentative set of geological divisions, based primarily on lithologic distinctions As we had no maps, we used only breast-pocket notebooks in which to record our observations; hence the ideas we formed as to the geological structure ... could not be fully put on paper until the topographer’s notes for the whole area surveyed had been platted and engraved, and our tentative geological columns modified or confirmed by the determination of our fossil collections by specialists.[This is why it took two years to prepare maps after field work was completed.]

Primitive tools and lack of information were not the only reasons that “conflicting statements were made” by the three Surveys. Competition and growing friction among the leaders were to blame as well.

When, in the winter of 1874-1875, the working up of our material was so far advanced that we could draw in the geological outlines on the Green River sheet, there was a question as to what color should be given to the great quartzite core of the Uintas. ... To correlate it with this formation [Precambrian rocks in the Wasatch Mountains]involved the assumption of an unconformity ... no such unconformity had been observed, however, and one of the rules laid down by Mr King for geological mapping was not to represent any such features as faults or unconformities which were not proved by actual evidence in the field. Hence,with some misgivings, but as the only alternative, the doubtful quartzite was correlated with the Weber, and so mapped. That such an unconformity had actually been observed by Mr Powell in the deep canyons of Green River, which we had been unable to explore through want of boats, was only known to us some years later.

"The doubtful quartzite" with Utah juniper, pinyon and rabbitbrush.

In other words Powell had notshared with other survey geologists what he had seen in the canyons of the eastern Uintas. Emmons was especially upset because he had discussed Uinta geology with Powell after the 1871 field season ... at Powell’s request.

Mr. Powell paid me the compliment of asking me to explain to him my ideas of the Uinta structure, saying that his observations in his boat journeys down the Colorado being necessarily confined to the immediate vicinity of the river, were difficult to coordinate, and he felt it would be of great help to him in working them out to thoroughly understand the Uinta uplift, which was evidently the key to the whole section. At that time he did not mention the unconformity by erosion ... but in other respects we compared notes freely and agreed in all essential points.

The acrimony intensified when King and Emmons found they were in a race to publish, with Powell in the lead.

after he [Powell] had borrowed a proof-sheet of our topography of that region, it was learned that he proposed to publish a volume on the Uinta mountains in the near future, it was realized that he would probably secure a priority of publication over us.

In a vain attempt to obviate this, Mr King, on November 15, 1875, caused 12 printed copies of the geologically colored Green River basin map, signed and dated in his own handwriting, to be distributed to as many of the leading geologists in the country. When the Powell report appeared (without any reference to our previous studies) the most important difference ... was as to the age of the Uinta quartzites.

But in the end it hardly mattered. All of the Great Survey geologists were incorrect about the Uinta sandstone, though that's understandable given the times. They all made major contributions to knowledge of geology and the American West, yet it wasn’t long before their reports were gathering dust and their maps were obsolete. As always, science marches on.

Their wonderful maps, compiled after so much physical labor, are now useless, save to historians and collectors. Richard Bartlett on the Great Surveys (1962)

Map of Hayden surveys; from Special Collections, University of Wyoming Libraries.

Hayden, FV. 1871. Preliminary report of the United State Geological Survey of Wyoming, and portions of contiguous territories. Washington: Government Printing Office. [footnote 3]

Powell, JW. 1876. Report on the geology of the eastern portion of the Uinta Mountains and a region of country adjacent thereto. [and accompanying Atlas] Washington: Government Printing Office. [footnote 1]

15,000 square miles in a single field season...I never stop being amazed at how those early expeditions did what they did, in attempting to figure out the geology. This, together with Clarence Dutton's description of the Great Basin ranges as 'an army of caterpillars," I stand humbled. Thanks for helping to put things into even more perspective. Excellent post.